Does your camera have the ability to spot meter?? If so, spot meter on the water and you will prevent blown highlights. This however will cause the area under the bridge to be underexposed, requiring some selective post work to bring out the shadow detail.

Another easier method would be to take this pic on a cloudy/overcast day, where you have no direct sun and the two 2 areas (under the bridge and the waterfall) will be chave similar exposure requirements.

That's a pretty good pic, bro'... Except for the waterfall.
Yeah, I agee with the others that your only good option is to reshoot the scene without blowing the water.
If you can't do that -- like the scene was taken 2000 miles away on a vacation -- you possibly could select the blown water and use the color picker tool to sample the color of the "good" water and fill the selection with that color. To add a bit of charactor you might then clone some of the texture from the "good" water into the waterfall and then use something like the motion blur tool to stretch the cloned texture out to mimic downward water flow.
Admittedly, this stuff is kind of Rube Goldburg, but only you can decide how much effort you want to put into "fixing" the problem.Grant

Do you know what? That shot has some serious potential and I personally would keep trying until I got it spot on. It would look great as a greetings card for example.

I agree with Bill. I would approach this with a merge of 2 shots. Try getting the outside right first and worry about the inside later. It is easy to pull and enhance detail from dark areas but when you have blown out highlights, forget it!

Use a tripod, stay dead still in between shots and take one shot exposed for the waterfall, another for the interior and blend them.

I hope you don´t mind my playing with your pic but this took less than a minute to get your picture to this stage, so I don´t think you are far off.